A number of different technologies have been used to provide electronic displays with paper-like appearance. For example, one class of paper-like displays includes electrophoretic displays, which use electrical fields to move “electronic ink” relative to a background. An electrophoretic display, in one configuration, fills a volume between a transparent viewing plate and a background plate with a liquid containing a dark dye and light colored particles. Charging agents cause the particles to hold a charge, so that voltages locally applied to pixel areas of the plates cause the light colored particles to move closer to or further from the viewing plate. The pixel areas in which the light colored particles collect near the viewing plate then appear lighter than pixels where the lighter particles are repelled from the viewing plate. Such displays may provide superior image quality, in particular a wider viewing angle and higher contrast, in some applications when compared to current LCD and plasma displays. However, current paper-like displays have several disadvantages or challenges that must be overcome to obtain greater commercial success.
One disadvantage is that many paper-like display technologies that are currently under development would require high current and/or high power to operate at video rates. In particular, many paper-like displays must switch a large volume of material or chromophores from one state to another to produce an adequate change in the optical properties of a pixel. For example, typical dye molecules have extinction coefficients on the order of 105 M−1 cm−1 or less, requiring about 1016 or more molecules per square centimeter of a display in order to absorb sufficient light in the absorptive state. At video switching rates, currents on the order of hundreds of mA/cm2 are needed if a unit charge must be delivered to each dye molecule to affect the change. Thus, display techniques that rely, for example, on redox reactions to switch dye molecules require unacceptably high currents for displaying video. The same holds true for electrochromic displays.
Another disadvantage is the slow speed that current paper-like displays typically provide. In particular, many existing paper-like display technologies involve phenomena that are intrinsically slow. For example, some electrophoretic or electrochemical techniques require species/particles to diffuse or drift through fluids over distances that create a slow response.
Another difficulty for current paper-like displays is achieving high quality color. In particular, most paper-like display technologies can only produce binary color from one material set (e.g., switch from one fixed color to another fixed color or from one fixed color to either black or white). Because of this, at least three sub-pixels using different material sets must be used when employing a side-by-side sub-pixel architecture with fixed colors (e.g., red-green-blue or cyan-yellow-magenta). This limits the maximum fraction of reflected light for some colors to about ⅓, so that the pixels of this type cannot produce saturated colors with good contrast. The alternative is to use a stacked architecture, but this also limits the achievable reflectivity and contrast because of the large number of layers required. Additionally, the required stacked architectures for color pixels can be complicated and difficult to manufacture. In particular, systems with a stacked geometry using active layers that can achieve only certain fixed colors generally require a minimum of four active layers (e.g., CYMK) and associated backplane electronics. This complexity impedes performance, increases manufacturing costs, and lowers yields. In addition, some active structures are particularly difficult to manufacture such as stacked reservoirs for electro-wetting devices. Finally, some reflective pixel technologies, such as front-back electrophoretic devices, cannot be stacked because the active layers cannot be put into a transparent state.
Another disadvantage of some current paper-like displays is their limited useful life. In particular, to sustain video rate operation for a period of years requires at least 109 reversible changes of optical properties even for a relatively low duty cycle. Achieving the desired number of cycles is particularly difficult in paper-like displays using techniques based on chemical reactions such as redox reactions, or techniques that involve mixing and separation of species.
In view of the current limitations of paper-like display technologies, better systems and methods for producing and operating paper-like displays are desired.